1. Field of the Invention
The present invention relates to ceramic slurry compositions and methods for producing ceramic green sheets and multilayer ceramic electronic devices, and more particularly, relates to a ceramic slurry composition for use in production of ceramic electronic devices, such as multilayer ceramic capacitors and multilayer ceramic substrates, and to methods for producing a ceramic green sheet using the ceramic slurry composition and for producing a multilayer ceramic electronic device using the ceramic green sheets.
2. Description of the Related Art
Multilayer ceramic electronic devices, such as multilayer ceramic capacitors, and multilayer ceramic substrates, are generally produced by the steps of laminating ceramic green sheets, compressing the laminate and heating for sintering the ceramics and electrodes.
For example, when a multilayer ceramic capacitor as shown in FIG. 1 is produced in which internal electrodes 2 are formed in a ceramic element 1, and a pair of external electrodes 3a and 3b are formed at two side surfaces of the ceramic element 1 so as to be connected with the internal electrodes which alternately extend to one side surface and to the other side surface of the ceramic element 1, the method described below is generally used.
(1) Electrode-provided sheets 11 (see FIG. 2) are formed by disposing internal electrodes to be used as electrodes for a capacitor on the green sheets produced by the method mentioned above.
(2) A predetermined number of the electrode-provided sheets 11 are laminated as shown in FIG. 2, ceramic green sheets having no internal electrodes thereon (sheets used as outer layers) are disposed on the top and the bottom of the laminated sheets, and they are compressed, whereby a laminate (a compressed laminate) is formed in which ends of the internal electrodes 2 alternately extend to one side surface and to the other side surface of the laminate.
(3) The laminate is baked under predetermined conditions so as to be sintered, and an electroconductive paste is coated on two side surfaces of the baked laminate (the ceramic element 1) (see FIG. 1) and is baked, whereby the external electrodes 3a and 3b which are connected with the internal electrodes 2 are formed.
Accordingly, a multilayer ceramic capacitor as shown in FIG. 1 is produced.
Other multilayer ceramic electronic devices, such as multilayer ceramic substrates, are also produced by the step of laminating ceramic green sheets.
Ceramic green sheets for use in the production of multilayer ceramic electronic devices are generally formed by steps of preparing starting materials, such as a powdered ceramic, a dispersing medium (e.g., a solvent), a dispersing agent, a binder, and a plasticizer, so as to produce a predetermined composition; mixing and pulverizing the starting materials thus prepared by using a media-type mill, such as a bead mill, a ball mill, an attritor, a paint shaker and a sand mill, so as to form a ceramic slurry; molding the ceramic slurry into sheets having a predetermined thickness by methods such as the doctor blade method; and subsequently, drying the sheets thus formed. In this connection, the media-type mill mentioned above is an apparatus for dispersing a powdered ceramic between media by mixing and stirring the powdered ceramic with the media.
However, recently, miniaturization and improved performance have been required for various multilayer ceramic electronic devices, such as multilayer ceramic capacitors, as is the case with other electronic devices.
Accordingly, ceramic green sheets for use in the production of multilayer ceramic electronic devices must be thinner, and recently, use of extremely thin ceramic green sheets of 10 μm or less thick is increasingly necessary.
When the extremely thin ceramic green sheets described above are produced, a ceramic slurry used for production of ceramic green sheets must be used in which the starting powdered ceramic is sufficiently dispersed. Hence, as a starting powdered ceramic, a fine powdered ceramic having an average particle diameter of 0.01 to 1 μm must be used.
However, in a ceramic slurry used for the production of the ceramic green sheet as described above, in general, a dispersing agent is conventionally used which is a lower molecular compound of a binder in consideration of the compatibility with the binder.
That is, as a binder, polyvinyl butyral resins, cellulose resins, acrylic resins, vinyl acetate resins, polyvinyl alcohol resins and the like are often used, and hence, lower molecular compounds of the binder mentioned above are generally used as a dispersing agent.
In this connection, most of the resins used as binders, such as polyvinyl butyral resins, cellulose resins, acrylic resins, vinyl acetate resins and polyvinyl alcohol resins, are nonionic compounds, and as a result, the low molecular resins thereof used as dispersing agents are also nonionic compounds.
The nonionic dispersing agents mentioned above have low adsorbing rates on powdered ceramics, and hence, a fine powdered ceramic having particles of 1 μm or less in diameter, which has strong cohesive force, cannot be rapidly and efficiently dispersed. Consequently, there are problems in that serious damage is done to the powdered ceramic and the productivity is decreased due to the longer time required in the dispersing step.